This invention relates to a process for metallic coat finishing, having good gloss vividness, particularly metallic glossy effect, by a so-called two coats-one bake system.
For the purpose of prevention of environmental pollution the coat finishing by a non-solvent, powder paint has rapidly been developing in recent years. However, in the coat finishing using only the powder paint having metal powders or colored pigments incorporated therein, owing to insufficient orientation of the metal powders in the coated film it is very difficult to obtain the coat of gloss vividness and smoothness equal to conventional solvent type metallic coatings. Therefore this coat finishing is not yet used in practice.
The gloss vividness used herein means a combined appearance of visible glossy effect and metallic glossy effect when the coat is observed by the naked eyes.
In case of using the conventional solvent type metallic coatings or aqueous metallic coatings, a coat finishing by applying the metallic coating, then effecting a pre-drying or bake-hardening, overcoating a thermosetting, clear powder paint and baking, i.e. a so-called two coats-two bakes system is disclosed in, for example, U.S. Pat. No. 4,142,018 and Japanese Patent Kokai No. 52-69446. This system, however, requires another baking furnace because of the two baking steps and also, is uneconomical in view of saving the energy.
For solving the above disadvantages, a coat finishing by applying the conventional metallic coating, then overcoating a thermosetting, clear powder paint and bake-hardening, i.e. a so-called two coats-one bake system is disclosed in, for example, Japanese Patent Kokai Nos. 54-4934 and 54-25943.
In the former a thermosetting resin containing as a cross-linking agent a monohydroxyl compound-denatured melamine-formaldehyde resin is used as a coated film forming component, whereas in the latter a high boiling solvent having a boiling point above the baking temperature minus 80.degree. C., its solubility parameter being within the range of .+-.1.2 of the solubility parameter of the resin in a top coat powder paint is used for a high boiling component of a solvent mixture contained in a solvent type thermosetting coating.
These methods, however, result in lowering of the gloss vividness and smoothness of the coat and cause serious problems in practical uses.